Understanding Flu Season

While we see flu activity at epidemic levels every year, some years are clearly much worse than others.

What kind of flu season will we have this year?

And while the severity of a flu season is easy to see after it is over, many of us could use a little help making sense of things when we are still in the middle of it.

Geographic Spread of Influenza Viruses – when you hear that there is widespread flu activity in a lot of states, this is what they are talking about. The only problem is that this doesn’t really tell you anything about the severity of a flu season. We get widespread flu activity in all states, or almost all states, each and every year as flu season peaks.

ILI Activity Indicator Map – like the maps showing the geographic spread of the flu, the ILI activity indicator map can help you tell where flu is spreading, but since it doesn’t necessarily represent the whole state, it can be misleading.

Influenza-like Illness Surveillance – the proportion of outpatient visits for influenza-like illness (ILI), or basically, how many people are going to the doctor with flu symptoms. Once we get above the national baseline of 2.2%, we know that flu season is starting in an area. How high can ILI get? Recently, it has peaked between 3.6% (2015-2016) and 7.5% (2017-2018). But that’s nationally. Because of wide variability in regional level data, you might see much higher ILI numbers in your state. For example, the regional baseline in Texas is 4%, while it is just 1.1% in Idaho. Still, widespread flu activity with a high ILI likely means a bad flu season.

Flu-Associated Hospitalizations – laboratory-confirmed influenza-associated hospitalizations. Overall, as flu season peaks, this can range from 4 to 5 per 100,000 population in a typical flu season, to 9 or 10 in a more severe flu season. This is also reported by age group. During a bad flu season, flu-associated hospitalizations will be high.

Mortality Surveillance – the proportion of deaths attributed to pneumonia and influenza. At some point in flu season, we get above an epidemic threshold and more people die with the flu, especially during a bad flu season.

Pediatric Deaths – pediatric influenza-associated deaths have been a nationally notifiable condition since the 2004 flu season and since then, on average, about 118 kids die with the flu each year. Last season was especially bad, with 185 pediatric flu deaths.

So how do you really know if it is a bad flu season?

Look for a high ILI%, high flu-associated hospitalizations, which will almost certainly be followed by a high mortality surveillance.

You also want to check viral surveillance data. Are the majority of influenza viruses being tested antigenically and genetically similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses? If not, that could mean a vaccine strain mismatch and a worse flu season.

And don’t be surprised by reports of widespread flu activity or rising ILI. That’s just flu season.